The separation of particulate solids from a solids-gas suspension phase has been practiced for many years. Removal of solids from a flue gas is one area in which particulate solids have been separated from a solids-gas suspension phase.
More recently, a large body of separation technology has been developed in the field of fluidized bed hydrocarbon cracking. Typically, heated particulate solids, either inert or catalytic, are contacted with a hydrocarbon feedstock in a reactor to vaporize the feedstock. Thereafter, the vaporized feedstock and particulate solids must be separated from the mixed particulate solids-vaporized gas suspension phase with the vaporized feedstock being sent on for further processing and the particulate solids being heated and recycled for delivery to the reactor. In addition, the vessels used to heat the particulate solids must have provisions to vent or discharge the combustion gases that result from heating the particulate solids, usually by combusting the carbon or coke formed on the particulate solids during the processing of hydrocarbons, usually by a cracking reaction. The vent is usually a flue through which the combustion gases flow with entrained particles. Because of the value of the particulate solids, particularly catalyst particles, and the environmental regulations limiting the amount of catalyst particles that can be discharged into the atmosphere, means must be provided to separate the particulate solids from the vent gas.
Various devices have been used to separate particulate solids entrained with the gas streams, such as hydrocarbon vapors, flue gases, air, or any other gas or mixture of gases. One commonly used separation device is the now conventional cyclone separator. Several cyclone separators may be used depending upon the amount of gas to be processed and often cyclone separators are used in series to obtain a maximum recovery of the particulate solids from the vaporized gas stream. The cyclone separators typically are vertically oriented and comprise a cylindrical separation section, a conical section extending from the bottom of the cylindrical separator section and a dipleg through which the separated solids are discharged for recycle or transfer to another processing unit or a vessel for further disposal or treatment.
Further, the process of recycling or returning particulate solids recovered by a cyclone or several cyclones to the original vessel encounters problems due to pressure differentials in the system. The pressure in the cyclone is lower than the pressure in the original vessel due to a pressure drop of the gas-solid suspension as it passes through the cyclone system. To enable the solids to flow from the cyclone, at a lower pressure point, to the original vessel, or any other vessel which is at a pressure higher than the pressure in the cyclone, the dipleg or pipe is connected at the bottom of the cyclone and the solids in it are kept in a fluidized state by the gas entrained with solids. If required, additional aeration with any suitable gaseous medium may be provided to keep the solids in the dipleg in a properly fluidized state. The static head built up by the column of fluidized solids in the dipleg overcomes the pressure differential between the cyclone and the vessel to which the fluidized solids are being transferred or returned, causing the solids to flow from the cyclone through the dipleg to the vessel.
To facilitate the transfer and flow of solids from the dipleg to the vessel, mechanical valves such as J-valves, flapper valves or trickle valves located at the terminal end of the dipleg are used. These valves provide a seal at the end of the dipleg to prevent gas from flowing from the vessel to the cyclone when circulation of solids through the cyclone has not been established.
The cyclone separators are known to be placed internal to the vessel which has a fluidized bed in which combustion or any other chemical reaction is taking place. For example, cyclone separators found in conventional fluid catalytic conversion (FCC) systems for cracking of hydrocarbons are generally located within the vessel assemblies, such as the regenerator or reactor/disengager. When the cyclones are placed internal to the vessel, return or recycle of the solids recovered by the cyclone, it is known in the art to position the dipleg vertically down from the cyclone to the fluidized bed. The dipleg end may be submerged in the fluidized bed in the vessel, or it may discharge the solids at the top of the bed.
However, in certain situations, it is advantageous to locate the cyclone external to the vessel from which entrainment is taking place. This may be the case when the temperature in the vessel is too high. Locating the cyclone external to the vessel removes the metallurgical constraints posed by the high temperature. For example, with the advent of high temperature FCC systems, locating the cyclones external to the regenerator gives the advantage of using carbon steel instead of stainless steel.
Unlike the situation where cyclones are placed internal to the vessel in which the entrainment occurs, the provision of exteriorly located cyclone separators requires the return of recovered catalyst to the system vessels through piping offset from the system vessel. It would therefore be a notable advance in the state of the art if an apparatus and process could be provided with a means for returning particulate solids from an external cyclone to a desired piece of system equipment which would result in a smooth transfer of fluidized solids without the possibility of reverse flow or clog.